The present invention relates to a transmitter/receiver for scrambled signals and, more particularly, to a transmitter/receiver for scrambled signals in radio or cable broadcasting systems in which only specific subscribers can view the contracted programs.
The CATV systems, for example, has developed for distibuting mainly TV programs. Recent demands in this field, however, involve qualified and varied services revolving around the transmission of sounds of high fidelity and data in addition to the TV program. A desirable system for satisfying such demands is a pay program system in which a viewer selects special program services according to his taste or desire and pays the broadcasting or cablecasting station for program services for his viewing or hearing. Accordingly, the subscribers are categorized into two types, one is a general subscriber, and the other is a special subscriber who desires such qualified and varied services.
In a broadcasting system based on the pay program system, it is necessary to scramble the pay program signal in order to prevent the general subscribers from viewing or hearing the pay program. Conversely, the key data to descramble the scrambled program must be given to the special subscribers so as to ensure that the special program services reach the special subscribers.
A prior art pay program system is disclosed in Japanese Publication No. 57-30438 (U.S. application Ser. No. 160985). In this system, the key data is scrambled two times, that is, the key data is scrambled again by other key data. Further, in this system the scrambled key data is sent to the special subscribers. The scrambled key data is decoded by the contracted subscriber's unit so as to descramble the pay program. As described above, the ability to enhance the secrecy of the pay program and the key data is improved in such a system. However, further improvements in the ability to enhance the secrecy of the pay program and the key data is further demanded. For this purpose, the present inventor hereby proposes to alter the contents of the key data in a periodical manner.
The alteration of the key data involves rewriting the data in a random access memory (RAM) of the subscriber's processing unit. In this approach, however, another problem arises in that during the transition period used for rewriting the key data, the previous key data is invalidated by the station, and new key data is written into the RAM. In this case, although some of the subscribers have contracted with the station for viewing the program, they fail to view the program. This problem occurs when there are a great number of contracted subscribers and it takes an overly long time for the station to rewrite the key data of each subscriber. Furthermore, a subscriber who contracts immediately after the station replaces the old key data with new data fails to view the program when he first powers on his processing unit.